It will work with 6.2.8 as fmw42 says: "The arc effect requires IM 6.3.5-5"

Your code works for me ( 6.4.8 ) no error about a missing variable so it MUST be something wrong with your setup. Talk to your host; find a friend who has a website and get them to try it, upload the code again to make sure you have not changed something by accident.

For those following this topic. The texteffect script was created with IM v6.3.5 and will not run on IM 6.2.8 as is due to the use fx calculations that need to be ported to bc calculations in order for the script to work. Apparently the fx calculations do not work correctly on such an old version of IM as 6.2.8.